DIGITAL IMAGE PROCESSING Quiz exercises preparation for the midterm exam

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1 DIGITAL IMAGE PROCESSING Quiz exercises preparation for the midterm exam In the following set of questions, there are, possibly, multiple correct answers (1, 2, 3 or 4). Mark the answers you consider correct. 1. If the spectrum of a continous (not sampled) image is the one in Fig. 1.a), then the spectrum of its sampled version is, most likely: a) the one in Fig. 1.b); b) the one in Fig. 1.c); c) the one in Fig. 1.a); d) the one in Fig. 1.d). Fig. 1.a) Fig. 1.b) Fig. 1.c) Fig. 1.d) 2. A sampled (but not quantized) image, whose brightness before quantization, in each spatial position, can take values in the range [0mV;250mV], has the linear histogram of its brightness represented (approximately) in Fig. 2.a). After the quantization, the histogram of the resulting digital image is the one in Fig. 2.b). Then, most likely, the quantizer that was used is: a) an 8 bit uniform quantizer; b) a 2 bit optimal quantizer; c) a 4 bit optimal quantizer; d) a 2 bit uniform quantizer. Fig. 2.a) Fig. 2.b)

2 3. If the result of the uniform quantization at 2 bits (4 quantization levels) of a sampled image is shown in Fig. 3.a), then the result of the quantization of the same image with the same quantizer, but using the pseudo-random noise quantization technique, is most likely: a) The same as the image in Fig. 3.a), just with more noise added; b) The one in Fig. 3.b), because in such a scheme, the noise is subtracted after quantization; c) The one in Fig. 3.c); d) The one in Fig. 3.d). Fig. 3.a) Fig. 3.b) Fig. 3.c) Fig. 3.d) 4. A uniform 2-bits quantizer for the input brightness range [0;220][mV] must have: a) the decision levels: t 0 =0; t 1 =55; t 2 =110; t 3 =165; t 4 =220; b) the decision levels: t 0 =00; t 1 =01; t 2 =10; t 3 =11; c) the reconstruction levels: r 0 =27.5; r 1 =82.5; r 2 =137.5; r 3 =192.5; d) the reconstruction levels: r 0 =0; r 1 =27.5; r 2 =82.5; r 3 =137.5; r 4 =192.5; r 5 =220.

3 5. Consider the original image from Fig. 4.a) (an original image affected by salt and pepper noise). Most likely, the Fourier amplitude spectrum of this image will look: a) as in Fig. 4.b); b) as in Fig. 4.c); c) as in Fig. 4.d); d) as in Fig. 4.e). Fig. 4.a) Fig. 4.b) Fig. 4.c) Fig. 4.d) Fig. 4.e)

4 6. The greyscale clipping function in the right is applied on an image having the histogram in Fig. 5.a). Then, most likely, the histogram of the processed image will look: a) like in Fig. 5.b); b) like in Fig. 5.c); c) like in Fig. 5.d); d) the same as before processing, since we have a linear function. Fig. 5.a) Fig. 5.b) Fig. 5.c) Fig. 5.d) 7. In order to obtain the image in Fig. 6.b) from the original image in Fig. 6.a), the following point processing operation should be applied: a) contrast compression; b) negativation; c) histogram equalization; d) histogram modification.

5 Fig. 6.a Fig. 6.b. 8. On the original grey scale image from Fig. 7. a), which of the following point processing operations could have been applied to obtain the image in Fig. 7.b)? a) Contrast compression; b) Negativation (image inversion); c) Some grey scale slicing operation; d) Extraction of the most significant bit. Fig. 7.a Fig. 7.b 9. Fig. 8.a) represents the grey level histogram of a digital image. After processing this image, one gets another grey level digital image with the grey level histogram shown in Fig. 8.b). What is the most plausible processing applied on the original image from the ones below? a) Grey scale inversion (negative of the original image); b) Binary thresholding; c) Histogram equalization; d) Some grey scale slicing. Fig. 8.a) Fig. 8.b) 10. The image in Fig. 9.a) represents an original image of low contrast, having the linear grey level histogram given in Fig. 9.b). If on this image, a point processing (grey scale transformation) is applied, using the transfer function from Fig. 9.c), which of the following will be the resulting image? a) the same as the original (Fig. 9.a)); b) the image in Fig. 9.d);

6 c) the image in Fig. 9.e); d) the image in Fig. 9.f). Fig. 9.a) Fig. 9.b) Processed level Fig. 9.c) Original level Fig. 9.d) Fig. 9.e) Fig. 9.f)

7 Quiz solutions 1. According to the 2-D sampling theory, the spectrum of the sampled image is obtained as a set of scaled replicas of the original image spectrum, placed around multiples of the sampling frequencies, horizontally and vertically. Therefore, since the only image in Fig. 1 matching this theoretical knowledge is Fig. 1.c) (which looks like a periodical repetition of some scaled version of the original image spectrum shown in Fig. 1.a)), it means the only possible correct answer is b). 2. Let us examine the two figures (Fig. 2.a) the histogram of the sampled, but non-quantized image, and Fig. 2.b) the histogram of the sampled and quantized image), and the possible options for the most plausible quantizer used. Fig. 2.a) Fig. 2.b) The first answer, a), sais the quantizer used is an 8 bit uniform quantizer; this would mean we have 2 8 =256 quantization levels 256 possible grey levels in the output image (therefore, also 256 possible bins in the resulting histogram), and 256 quantization intervals of equal width. But if such a quantizer would have been used, since the non-quantized brightness range is [0mV;250mV], it would mean we would have the width of each quantization interval of 250/256 = mv. Since the histogram of the nonquantized image contains data at least in the range [50mV;225mV], it means we would have use much more quantization intervals (therefore much more quantization levels) in the output, quantized image, than just 4 levels as seen in the histogram of the quantized image in Figure 2.b). Therefore, the first answer cannot be the correct answer. The second answer, b), sais the quantizer used is a 2 bit optimal quantizer; a 2 bit quantizer would give 2 2 = 4 quantization levels 4 possible grey levels in the output image (therefore, also 4 possible bins in the resulting histogram) as it is the case in the histogram of the quantized image, where we indeed have 4 bins. Furthermore, if the quantizer is optimal, the width of the quantization intervals does not have to be the same for the entire signal range, and therefore also the quantization levels (which are placed exactly in the middle of the quantization intervals) are usually not equally spaced. This also matches the histogram observed in Fig. 2.b). Therefore the second answer is correct, it is possible to use a 2 bit optimal quantizer and to obtain a quantized image having the histogram as in Fig. 2.b) The third answer, c), sais the quantizer used is a 4 bit optimal quantizer. In that case, we would have 2 4 = 16 quantization intervals and 16 quantization levels available. Again, as in the first case (option a) ), it is impossible in that case to obtain only the four bins in the output histogram, since the quantization intervals would have the width: 250/16 = mV, therefore the quantization intervals would be: [0;15.625), [15.625; 31.25), [31.25; ), [46.875; 62.5), [62.5; ), [78.125; 93.75)..., which means, so far, that we should have at least two bins in the output histogram corresponding to the range [50;100], and this is obviously not the case in Fig. 2.b). Therefore the quantizer used couldn t be a 4 bit uniform quantizer. Finally, the last answer, d), which indicates as possible a 2 bit uniform quantizer, cannot be correct, since this would mean the 4 bins in the histogram in Fig. 2.c) (the quantization levels) should be equally spaced (as they should be in the uniform quantizer s case) and is obvious that they are not.

8 Therefore the only possible (correct) answer is b). 3. According to the theory of uniform quantization using pseudo-random noise, the block diagram of the quantizer looks as in the figure below: u(m,n) v(m,n) 2 bits uniform v (m,n) Σ quantizer Σ η (m,n) Uniformly distributed pseudo-random noise, [-A,A] - u (m,n) Since a pseudo-random noise is first added to the input image prior to quantization, and then subtracted from the output image after its quantization, it means we can expect some amount of noise to still be visible in the output image. Therefore, the quantized image cannot look like in Figure 3.b), since this is a noise-free image (it actually looks exactly like the original grey level Peppers image). Therefore, the answer b) cannot be correct. The first answer, a), again cannot be correct, since the pseudo-random noise is applied on the original image prior to quantization; therefore, the brightness in each point changes prior to its input to the quantizer, and this brightness is quantized, and of course, quantizing u ( m, n) + η ( m, n) is not the same as the sum of the quantized u ( m, n) and not quantized η ( m, n) (noise). As possible answers, we still have c) and d). By examining Fig. 3.d), the image looks like the one in Fig. 3.a), just with some impulse noise added; this is not the usual case for a pseudorandom noise quantization scheme, where the noise level is much smaller, and its spatial distribution is more frequent, to allow the desired effect of this quantization scheme, that is: creating variations around the decision thresholds of the quantizer, to avoid for the false contours to appear in the quantized image. As one can see in Fig. 3.d), we still have the false contours, just that we also have some high amplitude noise, thus not achieving the desired goal. Therefore, the answer d) is not correct either. However, the image in Fig. 3.c) looks exactly as one would expect from a 2-bits quantizer with pseudorandom noise: the false contours disappear, there is some noise uniformly distributed on the image (due to its addition prior to quantization and its subtraction after the quantization from the quantized version of the image, one can see more than 4 grey levels present in the output image), but the amplitude of the noise is rather small as compared to the grey level dynamics of the original image. Thus, the correct answer is c): the result of the quantization of the image using a 2 bits uniform quantizer in a pseudorandom noise quantization scheme is, most likely, the image in Fig. 3.c). 4. For a uniform 2-bits quantizer for the input brightness range [0;220][mV], the decision levels and the reconstruction levels are uniformly spaced over the input brightness range. The decision levels of the quantizer define the intervals of input values which are mapped after the quantization to the reconstruction levels, and each reconstruction level is found exactly in the middle of an interval (in the middle between two successive decision levels), as illustrated in the following figure.

9 The decision levels are typically denoted by t k, k=0,1,...,l, where L is the number of decision intervals of the quantizer (which of course equals the number of reconstruction levels of the quantizer). For the quantizer given in this exercise, since it is a 2-bits quantizer, it will allow for 2 2 = 4 reconstruction levels and 4 decision intervals, therefore, L=4. This means we will have 5 decision levels (which are needed to define the 4 decision intervals), denoted here by t 0, t 1, t 2, t 3, t 4, and 4 reconstruction levels, denoted here by r 0, r 1, r 2, r 3. Therefore, even just judging by the number of decision levels, one can see that the second option of the quiz, answer b), cannot be correct, since it states that there are only four decision levels instead of five; with four decision levels, one can only define three decision intervals, thus not enough for the given quantizer. By the same reasoning, the fourth option, d), cannot be correct either, since it states that we should have six reconstruction levels, which is impossible for a 2-bits quantizer (this only allows for four reconstruction levels, L=4). Thus, the only candidates as correct answers are the options a) and c). In order to decide if they are correct, let us compute the decision levels and the reconstruction levels of the 2-bits uniform quantizer for the input range [0;220][mV]. The equations that give the expressions of the decision levels and of the reconstruction levels of the uniform quantizer are the following: t t q = L 0, L tk = tk 1 + q, k = 1,2,..., L; t0 = the left value of the input data range, tl = the right value of the input data range, rk = tk + q, 2 k = 0,1,2,..., L 1. Therefore, in our case, t 0 =0; t L =t 4 =220; q=220/4=55, and the other values result as: t 1 =t 0 +q=0+55=55; t 2 =t 1 +q=55+55=110; t 3 =t 2 +q=110+55=165; t 4 =t 3 +q=165+55=220, - normal, since t 4 =t L =the right value of the input data range These are exactly the values for the decision levels given by the answer a) of the quiz, thus, the answer a) is correct. Using the equation for r k, we compute also the reconstruction levels: r 0 =t 0 +q/2=0+55/2=27.5;

10 r 1 =t 1 +q/2=55+55/2= =82.5; r 2 =t 2 +q/2=110+55/2= =137.5; r 3 =t 3 +q/2=165+55/2= = But these are exactly the values for the reconstruction levels given by the answer c) of the quiz, thus, the answer c) is as well correct. This means that both the answers a) and c) of the quiz are correct, whereas the answers b) and d) are false. 5. Examining the original image in Fig. 4.a) (an original image affected by salt and pepper noise), one can expect that the noise component will introduce some random high frequency details in the Fourier spectrum of the image. In respect to this observation, the spectrum in Fig. 4.b) does not correspond to the noise-affected image, but rather to a clean (noise-free) image, since it does not have noticeable components in the simultaneously high horizontal and high vertical frequencies: High horizontal and high vertical frequencies; no components in the spectrum in Fig. 4.b) => no noise Thus, the first answer, a), cannot be correct. However, the second spectrum, represented in Fig. 4.c), presents high frequency components (corresponding to high horizontal and vertical frequencies), thus the second answer, b), is correct. The same Fourier spectrum (depicted in Fig. 4.c)) is also represented in Fig. 4.e), with the only difference that in the latter case, the representation of the Fourier spectrum considers the origin in the lower left corner, instead of the center (as in Fig. 4.c)); the four corners display the complex conjugate symmetry specific to the Fourier transform. Thus, the last answer, d), is also correct. However the third answer, c), indicating Fig. 4.d), is not correct, since obviously, in Fig. 4.d) we simply have the result of an edge detection applied to the noise-free version of the original image in Fig. 4.a). In conclusion, the answers b) and d) of the quiz are correct, whereas the answers a) and c) are false. 6. If one looks at the grey scale transformation presented, one can describe it as a grey scale clipping, with the following operation (see below):

11 - any grey level lower than 20 in the original image will be set to black; - any grey level above 60 in the original image will be set to white; - the contrast in the brightness range [20; 60] in the original image will be increased at maximum, since this input brightness range will be mapped on the output image in the brightness range [0; 255], according to the equation: 0, if u < 20 u 20 v = 255, if , if u > 60 u [ 20;60], where by u we denote the brightness in a certain spatial position in the original image, and by v the resulting brightness (after processing) in the same spatial position in the output (processed) image. Now, if we examine the histogram of the original image, presented in Fig. 5.a): we can see that there are no pixels in the brightness range [20; 60], and all the pixels of the original image are either under 20 (most of them, as we can see), or above 60! Therefore, the only brightness levels that will appear in the resulting image will be 0 and 255 (according to the analytical expression of the transformation, written above). This means that the histogram of the resulting image should have only two bins: one (with most pixels) on 0, and the other (with fewer pixels) on 255. Therefore, Fig. 5.b) and Fig. 5.c) cannot give the correct answer, since they have pixels on other grey levels too, and actually they have no pixels on 0. Neither the last answer (which states that the histogram of the processed image should be the same as before processing) is not correct, as we could see (anyway this answer couldn t have been correct unless we would have a single straight line as processing function, of slope 1, passing through the origin that is, unless the processing function could have been described by the equation v=u which is never the case in a grey scale clipping processing!). The only histogram that matches our expectations is the one in Fig. 5.d), therefore, the answer indicating Fig. 5.d) as histogram of the processed image is the only correct answer. Thus, the answers a), b) and d) are incorrect, and the answer c) is the correct answer of the quiz. 7. If we examine comparatively the images in Fig. 6.a) (original) and in Fig. 6.b) (after processing), we can see that the contrast in the processed image (that is, the difference between the lightest grey above the left flower, up near the image boundary and the darkest grey in the lower part of the image, between the two flowers, or, in the upper left corner of the image) is larger than the contrast in the original image; thus the processing should be one that can enhance the contrast, not decrease it. This means the first answer, a), which suggests contrast compression, cannot be correct. The second answer cannot be correct as well, since it states a negativation can be applied to yield from the image in Fig. 6.a), the image in Fig. 6.b); but negativation means that the lighter areas in Fig. 6.a) would become the darkest areas in Fig. 6.b), and vice-versa; this would mean, e.g., that the middle of the flowers

12 (which are lighter than the petals in Fig. 6.a)) would become darker than the petals in Fig. 6.b), and as we see, this is not the case: the middle of the flowers still stays lighter than the petals in Fig. 6.b) as well as in Fig. 6.a). Thus, b) is not correct. The third answer can indicate the correct processing: the histogram equalization, by its ability of spreading the grey levels widely across the whole brightness range from black to white, can have this contrast enhancement effect that is observed in Fig. 6.b) as compared to Fig. 6.a). Thus, the third answer, c), is correct. So is also the last answer, d), since histogram modification is just a variant of histogram equalization, leading to similar dynamic grey scale range modifications (i.e. possibly some contrast enhancement). Thus, the answers a) and b) are wrong, and the answers c) and d) are correct. Fig. 6.a Fig. 6.b. 8. Examining the original grey scale image (Fig. 7.a)) and the result of its processing (Fig. 7.b)), one can notice that the processed image is a binary (pure black and white) image. There are several ways to obtain such an image, so we should examine the possible answers for this quiz, to see which of them indicate a processing that could really lead to the image in Fig. 7.b) and which of them do not. Fig. 7.a Fig. 7.b The first answer indicates as possible operation, the contrast compression. This is definitely wrong, since the contrast of the second image (estimated as the difference between the lightest and the darkest brightness level in the image) is maximum (is actually exactly the difference from white to black), and probably higher than the contrast of the original image (or at least equal to the contrast of the original image). Thus, a) is an incorrect answer to this quiz. The second answer indicates negativation as possible operation; obviously this is again incorrect, because as a result of the negativation, the brightest areas in the image from Fig. 7.a) should become the darkest in Fig. 7.b), and vice-versa, which means, the apple should appear lighter than the background but this, as it can be seen, is not the case in Fig. 7.b) (the background becomes white from the brightest in Fig. 7.a), and the apple becomes black from the darkest in Fig. 7.a)). Thus, b) is also incorrect. Some grey scale slicing operation could indeed lead to the image in Fig. 7.b) from the image in Fig. 7.a), for suitably chosen parameters, as we saw earlier in Quiz 6: if the grey levels inside the apple are under the low threshold of the grey scale slicing function (to be set to black after processing) and the background grey levels are above the high threshold of the grey scale slicing function (to be set to white after processing), then the result would be exactly the one in Fig. 7.b). (In an extreme case, imagine one sets

13 the two thresholds equal then, one gets simply a thresholding operation, leading to a binary image as the one in Fig. 7.b) as a particular case of grey scale slicing). Thus, the answer c) is correct. The last answer, which states that one could apply the extraction of the most significant bit to obtain the image in Fig. 7.b), is also correct, since according to the definition of this algorithm, the result of processing any grey level u by this algorithm is the new brightness v=0 (black) if the most significant bit of u is 0, or v=255 (white) if the most significant bit of u is 1. Thus, considering an 8-bit representation of each brightness in the original image (which is the typical case for grey scale images), the examination of the most significant bit of the brightness means the examination of the value of u (the brightness) in comparison to 2 7 =128: if the most significant bit of u is 0, it means that u<2 7 =128. However, if the most significant bit of u is 1, then u 128 (equal only if only the most significant bit is 1, and all the others are 0). Thus, one can say that the extraction of the most significant bit is equivalent to a thresholding with a threshold of 128: v = 0, if 255, u < 128, if u 128 where by u we denote the brightness in a certain spatial position in the original image, and by v the resulting brightness (after processing) in the same spatial position in the output (processed) image. Such an operation can lead to the image in Fig. 7.b), starting from the original image in Fig. 7.a), so, the answer d) is correct as well. In conclusion, the answers c) and d) of the quiz are correct, whereas the answers a) and b) are false. 9. If the original image s histogram is the one in Fig. 8.a), and the resulting histogram after processing that image is the one in Fig. 8.b), then most likely, the processing applied is histogram equalization (which Fig. 8.a) Fig. 8.b) tends, ideally, to produce an image whose histogram is almost constant over the entire grey levels range). This corresponds to the answer c) of the quiz, thus answer c) is definitely correct. Let us examine now the other possible answers: the first one says that the processing was a negativation of the original image; this is not possible, because in this case, the histogram of the resulting image should be a mirrored version of the original histogram (with the mirroring done versus the vertical axis). This is because the negativation is a transformation of each grey level u in the original image in a new grey level v given by the relation: v = 255 u, if one considers the natural representation of the grey level images on 8 bits, thus, 0 corresponds to black and 255 corresponds to white (the maximum possible grey level in the image representation). Thus, whatever was darker becomes brighter and vice-versa, as a result of negativation, and basically, the number of pixels found on any grey level u in the original image move on the new grey level 255-u in the processed (negative) image leading to the mirroring effect of the histogram. Since this is not the case for the histograms in Figures 8.a) and 8.b), it means the answer a) is not correct.

14 The second answer is as well not correct; as a result of binary thresholding, the resulting image is a binary one, thus it contains only grey levels, and we should have only two grey levels with pixels (two nonzero bins) in the resulting histogram. As we can see from Fig. 8.b), there are many grey levels on which we have pixels (obviously much more than 2), therefore the processing cannot be thresholding, thus the answer b) is incorrect. We already saw that the third answer is correct; let s have a look on the last one. In the case of some grey scale slicing, one should have either an image containing just two grey levels (binary image) in which case the histogram obviously would not look like the one in Fig. 8.b), since only two bins would have non-zero height, either an image which keeps the grey levels unchanged within some brightness range, and sets the others to a constant grey (most commonly black or white). In the latter case however, one should find that portion of the original histogram in Fig. 8.a) over the plot in Fig. 8.b), and then, a constant grey level which would introduce a bin with a significant height (a large number of pixels on it), and as we can see, this is not the case of the histogram in Fig. 8.b). Therefore, the last answer, d), is not correct: the histogram in Fig. 8.b) does not correspond to an image obtained from the original one (with the histogram shown in Fig. 8.a)) by grey scale slicing. Thus, the answers a), b) and d) are incorrect, and the only correct answer of this quiz is c). 10. To derive the effect of the grey scale transformation whose plot is shown in Fig. 9.c) on the original image shown in Fig. 9.a), we should examine it in respect to the original image s histogram (shown in Fig. 9.b)). Looking on the grey level histogram, one can see that most of the grey levels in the original image fall in the range [155; 213]. On the other hand, examining the grey scale transformation plotted in Fig. 9.c), we see it represents a grey scale slicing described by the equation: 0, if u < 160 u 160 v = 255, if u [ 160;210], , if u > 210 where by u we denote the brightness in a certain spatial position in the original image, and by v the resulting brightness (after processing) in the same spatial position in the output (processed) image. This means that most of the pixels in the original image from Fig. 9.a) fall inside the range [160;210] which is linearly expanded by the grey scale clipping function to the range [0;255] (that is, from black to white), thus leading to a contrast enhancement effect on the processed image (by significantly increasing the difference between the brightest and the darkest grey levels in the resulting image, as compared to the original image). Let s recall that we are trying to see how will the resulting image look like, and we have as possible answers the following: (1) The image will look the same as the original this is not the case, since we just saw that our transform will produce the expansion of the grey level dynamics of our image in the range [0;255] from only [155;213]. Thus, the first answer, a), is not correct. (2) The image will look like the one in Fig. 9.d) but in this figure, if we look on the bridge crossing the river, we see that the bridge appears darker than the river; whereas in Fig. 9.a) (original), the bridge was brighter than the river, which means, the image in Fig. 9.d) implies also a negativation of the original image, which is not achievable by the grey scale clipping function shown in Fig. 9.c). So, the second answer, b), is incorrect as well. (3) The image will look like the one in Fig. 9.e) this is very likely; one can see that the bright grey levels (as e.g. the bridge, some buildings) become much brighter, the dark ones (as e.g. the river) become much darker, which means the contrast is enhanced, without any negativation. This matches the operation of the grey scale transformation from Fig. 9.c). Therefore, the third answer, c), is correct.

15 (4) The image will look like the one in Fig. 9.f) this answer cannot be correct, since the image in Fig. 9.f) has even lower contrast than the original image (which means, we can expect an even narrower histogram than the one in Fig. 9.b)) this does not match the grey scale processing function behavior derived above, which means the last answer is again not correct. Thus, the answers a), b) and d) are not correct, and the only correct answer of this quiz is c). Fig. 9.a) Fig. 9.b) Processed level Fig. 9.c) Original level Fig. 9.d)

16 Fig. 9.e) Fig. 9.f)